Coffee percolators of this type are for the most part used in the catering trade or canteens, although their use is not restricted thereto. They operate on the principle that the ground coffee is first supplied to the scalding chamber and poured onto the filter. Subsequently the scalding chamber is supplied with hot water impinging on the coffee meal layer therein and percolating therethrough to thereby extract the flavouring and other soluble substances from the ground coffee. The thus obtained coffee passes through the filter to be received in suitably positioned receptacles such as cups, dippers or reservoirs. After the coffee volume corresponding to the supplied amount of coffee meal has thus percolated, the hot water supply is discontinued, and the exhausted coffee-grounds are removed from the filter.
Subsequently fresh coffee meal is supplied in preparation of a further brewing operation. In a coffee percolator operating by this method, the employ of conventional filter papers or fabrics would be extremely troublesome and might lead to problems with regard to cleaning of the filter.
Known from DE-OS No. 21 32 596 is a coffee filter operating without the conventional filter paper. The coffee filter is of cup-shaped configuration, with both its flat bottom and its sidewall being formed of a metal sheet sieve with micro-perforations. The metal sheet sieve is formed as an integral component by a galvanoplastic process, i.e. by the deposition of a material from an electrolyte bath. The filter is secured to the bottom side of a hot water container by a bayonet coupling. The hot water container has a passage opening directly into the filter. In operation, the filter and hot water container are simply placed on a conventional coffee jug. After the proper amount of water has percolated through the filter, the latter has to be removed for manually cleaning it from the coffee-grounds in a flow of running water. The known filter is unsuitable for use in a coffee percolator having scalding chambers as employed for instance in the catering trade, because the introduction of the coffee meal and the removal of the coffee-grounds are rather troublesome operations. It is further known that coffee has rather corrosive properties. For this reason, and for reasons of hygiene, coffee should only be brought into contact with indifferent materials having corrosion-resilient properties with respect to coffee, such as glass, ceramics, certain synthetics, and stainless steel. None of these materials, however, are capable of being deposited by a galvanoplastic process. On the other hand, the formation of micro-perforations or slots is not feasible by mechanical processing such as drilling or cutting. Since in a coffee percolator having a scalding chamber the coffee is prepared at an elevated pressure, it is further required that the coffee filter have sufficient stability.